Sodar systems employ directed sound waves to detect atmospheric phenomena such as wind speed. By directing sound beams into the atmosphere in a number of directions, and measuring the Doppler shift of echoes returned from turbulence and discontinuities in the atmosphere, wind speed and other atmospheric phenomena can be accurately estimated. The predominant type of sodar in current use is the monostatic phased array sodar. Monostatic sodar systems emanate sound beams and listen for their reflections from a single location. Phased array monostatic sodars direct the beams in different directions, and are sensitive to echoes returned from these directions, by use of an array of sound transducers which transmit and receive in groups of differing phase so as to direct the transmitted sound beams and regions of sensitivity as desired.
Phased array monostatic sodars are often arranged with the phased array mounted vertically in a housing which transmits the beams in a generally horizontal direction. The beams impinge upon a flat sound reflective surface, or mirror, which is mounted at a generally 45 degree angle to horizontal. By means of this mirror, the beams are reflected upwards into the generally vertical direction. This arrangement, which is known prior art, allows the beams to be projected in the necessary directions, while substantially protecting the transducers comprising the array and the associated electronics from rain, snow and other forms of precipitation which might interfere with operation or even permanently damage the array system.
A shortcoming of this technique is that snow, sleet, and unevenly accumulating ice which may collect on the mirror can prevent the system from operating properly during, and following inclement winter weather. Prior art attempts at mirror de-icing systems have shortcomings which render them marginal at best.
One prior attempt at a mirror de-icing system used a propane radiant catalytic heater mounted beneath the mirror surface, in which the heater exhaust was vented directly into the sodar enclosure below the mirror. This system was inadequate due to the uneven distribution of the heat generated by the system. Snow melt was accomplished over the relatively small area directly warmed by the heater. Despite the relatively good thermal transfer properties of the aluminum mirror used, additional heat from the radiant heater was consumed boiling water in the area directly warmed, and heating the air above this area, while unacceptable quantities of ice and snow remained in other portions of the mirror. An additional shortcoming of this system was that the moisture generated when the radiant heater combusted propane was released into the lower housing of the instrument, where it condensed as water and ice on the rear surface of the mirror and side walls of the enclosure, saturating internal soundproofing material and providing a source of moisture which could be harmful to electronic equipment in the enclosure.
There is also a de-icing system with an electrical resistance heater mounted on the upper surface of the mirror. The quantity of energy required for an effective de-icing system is such that an electrical deicer cannot be expected to operate effectively using battery, or solar panel-and-battery power supply. This constraint thus forces the system to be installed with an internal-combustion engine based generator system, or with supply of utility power from an external source.